1. Field of the Invention
The invention relates to a wireless communication system, and more particularly, to a multi-band receiver utilized in a wireless communication system.
2. Description of the Prior Art
WLAN is a fast-developing and fast-changing technique both in its standard and its applications. For example, 802.11a utilizes a 5 GHz transmission band, but 802.11b and 802.11g utilize a 2.4 GHz transmission band. Furthermore, 802.11a has the disadvantage of short transmission distance and is inconvenient, because the 5 GHz transmission band is not available in some regions. Therefore, few products are designed which will only support 802.11a. The next generation of WLAN techniques and WLAN products will simultaneously support 802.11a and 802.11g so that the transmission efficiency can be raised and the number of users can be increased. Furthermore, the transmission quality demands can be met.
Nowadays, a dual-band/multi-band receiver comes in two categories. The first category establishes multiple receivers in a chip. In “A triple-band 900/1800/1900 MHz low-power image-reject front-end for GSM” ISSCC of Tech. Papers, pp. 408-409, Feb. 2001, a multi-band receiver is disclosed. Please refer to FIG. 1, which is a block diagram of the multi-band receiver 100 according to the prior art. As shown in FIG. 1, the multi-band receiver 100 comprises three single-band receivers 110, 120, and 130. Each single-band receiver 110, 120, 130 comprises a low-noise amplifier (LNA) 112, 122, 132 for receiving an RF signal RF1, RF2, RF3 (for example, 900 MHz, 1800 MNz, and 1900 MHz RF signals, respectively), a band-pass filter 114, 124, 134, and a mixer 116, 126, 136. Because three independent single-band receivers 110, 120, 130 are set up in a chip, the chip area is substantially occupied.
The second category utilizes a single circuit to achieve a multi-band receiver. For example, in “A SiGe low noise amplifier for 2.4/5.2/5.7 GHz WLAN applications”, IEEE international solid-state circuits conference, pp 364-365, San Francisco, USA February 2003, a multiple-band receiver is disclosed. The multi-band receiver is produced through an HBT producing procedure of SiGe. Please refer to FIG. 2, which is a diagram of an LNA of another receiver according to the prior art. In “Concurrent dual-band CMOS low noise amplifiers and receiver architectures, Symp.” on VLSI Circ. Dig., pp. 247-250, Jun. 2001, another receiver is disclosed. The LNA 200 successfully utilizes a CMOS producing procedure to achieve the purpose of “dual-band”. But as shown in FIG. 2, this circuit structure needs a lot of inductors to generate at least two central frequencies corresponding to dual-band, furthermore, because of the frequency response of the LC tank 220, the LNA 220 amplifiers produce unwanted noise when only receiving a signal with a specific frequency.
In U.S. Pat. Nos. 6,072,996 and 6,658,237, further multi-band receivers are disclosed. In these two patents, the multi-band receiver is achieved through establishing multiple receivers in a chip, and thus the details are omitted here.